Combustive destruction of halogenated compounds

ABSTRACT

The disposal of troublesome substances, especially global-warming halogenated compounds is difficult enough, but is particularly difficult when associated with particulate-forming matter, such as silane and arsine commonly encountered in waste gas streams of the semiconductor industry. The combustive destruction of the troublesome substances in such a waste gas stream is simply and successfully achieved by injecting the stream admixed with fuel gas into a combustion zone surrounded by the radiant surface of a foraminous gas burner that is separately fed fuel gas and excess air sufficient to burn all the combustibles entering the combustion zone. A simple apparatus integrates the combustion zone with a quenching zone for the combustion product stream.

BACKGROUND OF THE INVENTION

This invention relates to the disposal by combustive destruction oftroublesome substances, especially global-warming, air-pollutinghalogenated compounds, such as fluorocarbons, and particulate-formingmatter upon oxidation, such as silane.

Fluorocarbon gases such as C₂ F₆ and CF₄ are global-warming compoundswhen released into the atmosphere where they have extremely longlifetimes. These gases as well as other fluorinated gases such as NF₃and SF₆ are used in the manufacture of semiconductors during theetching, modification and construction of silicon wafers, and during thecleaning of the machines used in the process. Hydrides, such as silane(SiH₄) which ignites upon exposure to air, are also used in the processof making silicon wafers. The fluorinated gases and hydrides and evenparticulate matter are swept out of the machines with nitrogen,sometimes together and sometimes sequentially.

According to current practice, the nitrogen stream containing thetroublesome gases is subjected to thermal destruction using electricalheat or gas firing. However, complete destruction of the undesired gasesis achieved only with a large consumption of thermal energy. Anothercurrent technique of mixing the stream with hydrogen and effectingcombustion is unsatisfactory because of the large usage of expensivehydrogen.

It is significant that these expensive and unsatisfactory disposalmethods are in use even though several patents propose other procedures.For example, U.S. Pat. No. 4,627,388 burns halogenated hydrocarbon wastein a horizontal fire tube boiler requiring a refractory lined combustionchamber of substantial length to contain the flame front near adiabaticconditions. U.S. Pat. No. 4,206,711 uses a vertical combustion chamberwherein liquid waste is sprayed down from the top, while several flatflame radiation type burners in the walls of the chamber provide flamesthat totally surround the sprayed waste. U.S. Pat. No. 4,828,481eliminates the large and costly equipment of the aforesaid patents byproposing a combustion chamber comprising two opposed porous platesbetween which combustion is carried out. A mixture of gaseous fuel, air,and waste vapor is fed through one porous plate, burned in the chamber,and the combustion products are exhausted through the other porousplate. However, the waste material must be free of particles or theinlet porous plate will become plugged. Even in the absence of particlesin the waste material, there is the real danger that particles, such assoot or silica (if silane is in the waste), will form during combustionand plug the outlet porous plate. The need for a practical disposalsystem still exists.

In industrial practice, the gaseous stream carrying one or morehalogenated compounds may also contain particulate-forming matter uponoxidation, simultaneously or sequentially. Silane which oxidizes in airto silica, and another often used hydride, arsine (ASH₃) which oxidizesto a troublesome sticky oxide (As₂ O₃), are illustrative ofparticulate-forming matter commonly associated with halogenatedcompounds, particularly the fluorocarbons used in the semiconductorindustry.

Besides the fluorinated gases of the semiconductor industry, airpollutants encountered in other industries include chlorinatedhydrocarbons such as carbon tetrachloride, trichloroethylene,chlorobenzene and vinyl chloride. The refrigeration industry has longfavored chlorofluorohydrocarbons as refrigerant gases but these gasesare now being phased out of future use. A satisfactory system for thedisposal of all these halogenated compounds is still wanting.

A principal object of this invention is to provide a simple and economicsystem for the combustive destruction of halogenated compounds and/orparticulate-forming matter upon oxidation.

A further object is to provide an apparatus and a process that achievesubstantially complete combustive destruction of troublesome substanceswhile suppressing the formation of air pollutants, namely, nitrogenoxides (NO_(x)), carbon monoxide (CO) and unburned hydrocarbons (UHC),that are commonly formed during combustion.

Another important object is to utilize apparatus that is simple andeconomic to construct and operate.

These and other features and advantages of the invention will beapparent from the description which follows.

SUMMARY OF THE INVENTION

In accordance with this invention, troublesome substances, especiallyhalogenated compounds and particulate-forming matter upon oxidation aresubstantially completely (at least 95%) destroyed by combustion in aprocess comprising the steps of mixing a fuel gas with the streamcontaining troublesome substances and injecting the mixture into adestructive combustion zone maintained at a temperature of at least1,900° F. by effecting flameless combustion of a fuel and excess airmixture on the exit surface of a foraminous gas burner that surroundsthe combustion zone. The excess air passing through the foraminousburner is sufficient to consume not only the fuel supplied to the burnerbut also all the combustibles in the mixture injected directly into thedestructive combustion zone. Even then, there should be enough excessair so that free oxygen remains in the product gas stream leaving thecombustion zone. Generally, to achieve substantially complete (at least95%) combustion of the troublesome substances, the amount of excess airshould be at least about 10% more than the stoichiometric requirement toburn all the combustibles entering the combustion zone.

In most cases, natural gas is the most cost-effective fuel that can besupplied to the foraminous gas burner and separately admixed withhalogenated compounds and/or particulate-forming matter injected intothe destructive combustion zone. Other hydrocarbons and hydrogen arealternate fuels but generally are used only where natural gas is notavailable.

The foraminous gas burner used in accordance with this inventionincludes two basic forms: a porous fiber layer and a perforated plate.The porous fiber layer form involves a porous coherent layer ofnoncombustible fibers of either the mineral type or the metallic type.U.S. Pat. No. 3,179,156 to Weiss et al teaches the deposition ofalumina-silica fibers on a screen from an aqueous suspension of thefibers containing a binding agent which interconnects the fibers to oneanother and to the screen. This basic porous fiber burner made withceramic fibers preferably contains a small amount of aluminum powder astaught by U.S. Pat. No. 3,383,159 to Smith or aluminum alloy powder astaught by U.S. Pat. No. 4,746,287 to Lannutti. U.S. Pat. No. 3,173,470to Wright discloses a porous fiber burner in which a layer of metalfibers is made coherent by sintering. A recently developed hybrid fiberburner formed from a mixture of metal fibers and ceramic fibers istaught by U.S. Pat. No. 5,326,631 of Carswell et al.

The perforated plate form of foraminous gas burner is shown in numerouspatents. U.S. Pat. No. 2,775,294 to Schwank shows an early example of aperforated plate burner. Other forms of perforated plate burners areillustrated in U.S. Pat. No. 3,683,058 to Parriot and U.S. Pat. No.3,954,387 to Cooper.

All of the foregoing foraminous gas burners as well as variationsthereof widely described in patents and technical publications serve thepurposes of this invention.

The exit surface of the foraminous burner where flameless combustiontakes place surrounds the destructive combustion zone into which themixture of halogenated compounds and/or particulate-forming matter andadded fuel is injected. The resulting incandescent exit surface of theburner emits infrared radiation that helps to maintain the destructivecombustion zone at a temperature of at least 1,900° F. The fuel gas fedtogether with excess air to the foraminous burner is consumed byflameless combustion at the exit surface of the burner and the resultingcombustion product gas flowing outwardly from that surface prevents thedeposition thereon of particulate matter entering or forming in thedestructive combustion zone because of a hydride such as arsine injectedthereinto. The deposition of particulate matter at the exit of anyburner is troublesome and is extremely so when the particulate matter isa sticky substance such as As₂ O₃ formed when arsine is present in thestream injected into the destructive combustion zone. The selection offoraminous burners to prevent the deposition of particulate matterwithin the destructive combustion zone is essential to the successfulperformance of the invention.

In contrast to the flameless surface combustion of the fuel gas suppliedto the foraminous burner, the combustibles of the stream separatelyinjected into the destructive combustion zone burn with a diffusionflame. Inasmuch as the air required to burn the combustibles in theinjected stream is separately fed to the combustion zone through theporous fiber burner, it is prudent to inject that stream in the form ofseveral small streams so that the excess air leaving the exit surface ofthe foraminous burner can more rapidly react with the wastecombustibles. Clearly, it takes longer for the air to reach all of thecombustibles in a stream of large diameter than it does in severaldivisions of that stream. Stated another way, several small streams willhave a shorter flame than that of a single stream having a volume equalto the total volume of the several small streams.

The combustion product stream leaving the destructive combustion zone isnoteworthy for two reasons: at least 95% of the halogenated compoundsfed to the zone have been destroyed and the formation of NO_(x), CO andUHC have been suppressed to very low values. The combustion productstream will contain HF and HCl to the extent that fluorine and chlorinewere present in the halogenated compounds fed to the destructivecombustion zone. Oxide particles will be in the combustion productstream to the extent that hydrides such as silane and arsine werepresent in the stream entering the combustion zone. The product streamwill also contain any noncombustible particles present in the wastestream fed to the combustion zone.

The combustion product stream must be cooled and scrubbed to capture theHF and HCl and particulate matter present therein as well as sulfurdioxide if SF₆ was present in the stream of halogenated compounds. Asimple and effective way of cooling and even starting the capture of HFand HCl is to discharge the product stream from the combustion zonedirectly into a column in which a flow of water coats the inner surface.Spraying water into the product stream discharged from the destructivecombustion zone is also effective. The thus quenched product stream isthen passed through a scrubber which may be in any of its known forms.The scrubbed gas is vented to the atmosphere as an environmentally safegas.

BRIEF DESCRIPTION OF THE DRAWINGS

To facilitate the further description and understanding of theinvention, reference will be made to the accompanying drawings of which:

FIG. 1 is a sectional view of a rectangular pan-type porous fiber layerburner;

FIG. 2 is a diagrammatic horizontal sectional view of four burners ofFIG. 1 arranged to form a vertical furnace for the practice of theinvention; and

FIG. 3 is a diagrammatic sectional elevation of a preferred form offurnace, shown in association with desirable equipment for feeding thefurnace and for treating the gaseous effluent therefrom.

DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 is a cross-section of a pan-type porous fiber layer burner 10transverse to its length. Metal pan 11 has side walls 12 with screen 13welded to the ends 14 of side walls 12. A porous layer 15 of ceramicfibers is deposited on, and attached to, screen 13. The porous layer 15provides the exit surface at which a mixture of fuel gas and air willburn without visible flame and become radiant. The fuel gas-air mixtureis fed to burner 10 through pipe 16 connected to metal pan 11.

FIG. 2 forms a furnace 20 useful for the practice of this invention byhaving four porous surface burners 10 of FIG. 1 arranged to form asquare adiabatic combustion zone 21. Where each pair of burners 10 meetat right angles to one another, a refractory post 22 is cemented to theside walls 12 of the contiguous burners 10 so that the products ofcombustion cannot leak along the vertical (normal to FIG. 2) junctureline 23 of contiguous burners 10. By this arrangement, four burners 10act as an inwardly fired furnace with a destructive combustion zone 21surrounded by the exit surface 15 of burners 10. FIG. 2 demonstratesthat a furnace suitable for this invention may be formed of modularburners 10.

A waste stream containing halogenated compounds and/orparticulate-forming matter and added fuel gas enters the top of furnace20 through multiple openings 25 as small streams that flow down intocombustion zone 21 where excess air exiting from porous fiber layers 15achieves the combustive destruction of the troublesome compounds. It isunderstood that the bottom end of furnace 20 is open and connected to awater-cooling column for the capture, as previously explained, of anyHF, HCl, SO₂ and particles in the combustion product stream flowing fromcombustion zone 21.

A preferred furnace 30 is shown in FIG. 3 with desirable auxiliaryequipment for feeding a waste stream of halogenated compounds and/orparticulate-forming matter and added hydrocarbon gas to its destructivecombustion zone 31 and for treating the combustion product streamissuing therefrom. Furnace 30 is formed by cylindrical steel shell 32with flanges 33,34 at its opposite ends. Bottom flange 34 extendsinwardly and outwardly from shell 32. Cylindrical metal screen 35 havingits top connected to screen 36 and having inward screen flange 37connected to flange 34 is concentrically held within shell 30 and spacedtherefrom. Weld 35A fastens screen 35 to the inner edge of flange 34.Several tubes 38 extend through and are welded to top screen 36. All ofthe inner face of screen 35, 36, 37 has an adherent porous layer 39 ofceramic and/or metal fibers. Shell 32 has one or more pipes 40 forintroducing a mixture of fuel gas and excess air into the space aroundscreens 35, 36 so that the mixture will flow through porous fiber layer39 and, upon ignition, will maintain flameless combustion at the exitsurface of fiber layer 39.

Steel plate 41 supported by and attached to top flange 33 by bolts (notshown) holds as many tubes 42 as there are tubes 38 fastened to screen36 by weld 38A. Tubes 42 are of a smaller diameter than that of tubes 38and are long enough so that the bottom ends of tubes 42 reach the bottomends of tubes 38. The spacing of tubes 42 extending vertically throughplate 41 and welded thereto must be carefully laid out so that, whenplate 41 is brought down to rest on flange 33 of shell 32, each tube 42will slide through a tube 38. Any leakage of the gas-air mixture fromthe space above screen 36 through the clearance between concentric tubes38 and tubes 42 is generally tolerable. However, if desired, suchleakage can be easily stopped by a ring 43 of elastomer on each tube 42positioned to seat against the top end of tube 38 when tube 42 has beenfully inserted in tube 38.

Furnace 30 is connected to cooling column 50 so that the bottom open endof destructive combustion zone 31 is aligned with column 50 which has anannular trough 51 around its top end. Water is supplied to trough 51through pipe 52 and overflows the top end of column 50 to provide acontinuous flow of water down the inner surface of column 50 therebycooling the combustion product stream leaving zone 31 and preventingparticles in that stream from adhering to the inner surface of column50. The gaseous stream and water discharge from the bottom end of column50 into separator 54 having drain pipe 55 for the withdrawal of watercontaining particulate matter and soluble compounds such as HF, HCl, andSO₂. The cooled gaseous stream exits from separator 54 through pipe 56and is passed through a scrubber (not shown) in any of its many knownforms to capture residual soluble compounds in the gaseous streamleaving separator 54. The scrubbed gas is vented to the atmosphere as anenvironmentally safe exhaust.

A waste stream containing halogenated compounds and/orparticulate-forming matter is fed to tubes 42 while fuel gas is added tothat stream via tubes 44. The resulting mixture flows down tubes 42 intocombustion zone 31 where the combustibles thereof are consumed asseparate flames projecting from the bottom ends of tubes 42 upon meetingexcess air supplied through porous fiber layer 39.

Tests were conducted with a furnace having a cylindrical (3 inches indiameter and 12 inches long) destructive combustion zone surrounded by aporous ceramic fiber burner as illustrated by screen 35 and porous fiberlayer 39 in furnace 30 of FIG. 3. A nitrogen stream containing 8% byvolume of C₂ F₆ was injected into the combustion zone at the rate of 20liters per minute, while natural gas and excess air were passed throughthe porous fiber burner to effect surface combustion at the rate of42,000 BTU (British Thermal Unit) per hour per square foot of burnersurface. The excess air supplied to the burner was 61% more than thestoichiometric requirement of the natural gas simultaneously supplied tothe burner. The combustion product gas contained 11.3% by volume ofresidual oxygen. Analysis of this product gas revealed that only 56% ofthe C₂ F₆ injected into the furnace had been destroyed.

However, another test was carried out without making any changes in theforegoing operation except that 12% by volume (2.4 liters per minute) ofnatural gas was mixed with the nitrogen stream containing C₂ F₆. In thiscase, the excess air supplied to the burner also burned the natural gasadmixed with the nitrogen stream so that the residual oxygen in thecombustion product gas dropped to 7.4% by volume. Analysis of theproduct gas showed that 99% of the C₂ F₆ had been destroyed.

In another pair of tests, the burner was fired at the rate of 44,000 BTUper hour per square foot with 61% excess air and the flow of nitrogencontaining 4% by volume of C₂ F₆ was doubled to 40 liters per minute. Inone test, natural gas was added to the waste nitrogen stream at the rateof 2.4 liters per minute (6% by volume). The combustion product gascontained 7.1% by volume of residual oxygen. Only 60% of the C₂ F₆ wasdestroyed. Merely by increasing the addition of natural gas to 4.7liters per minute (11.8% by volume) the destruction of C₂ F₆ rose to 96%which is considered satisfactory when compared with current commercialprocesses that are more cumbersome and expensive. The residual oxygen inthe combustion product gas dropped to 4.4% by volume.

In still another test, the supply of nitrogen containing 2% by volume ofC₂ F₆ was quadrupled to 80 liters per minute and the burner was fired atthe rate of 58,000 BTU per hour per square foot with 61% excess air.Natural gas added to the nitrogen stream was 8.9% by volume (previoustest 12%). The combustion product gas contained only 3.5% by volume ofresidual oxygen. The destruction of 99% of the C₂ F₆ was achieved.

The first two tests demonstrate the critical need of supplying fuel gasadmixed with the waste stream injected into the destructive combustionzone. The second pair of tests shows that increasing the amount of fuelgas mixed with the waste halogenated compound stream increased thecombustive destruction of the halogenated compound. The last cited testmakes it clear that even though a waste stream with allow concentration(2% by volume) of C₂ F₆ was passed through the combustion zone at arapid rate (80 liters per minute), 99% of the fluorocarbon was stilldestroyed. The several tests demonstrate the broad range of flow ratesof waste stream with varying C₂ F₆ concentrations that can besuccessfully processed in a combustion zone surrounded by the radiantsurface of a foraminous gas burner.

The foregoing tests were carried out with C₂ F₆ free ofparticulate-forming matter such as silane and arsine to facilitate theanalysis of the combustion product gas to determine the percentage of C₂F₆ destruction achieved in each test. To establish what happens whensilane enters the destructive combustion zone, a test was conducted inwhich silane was added to a nitrogen stream injected into the combustionzone at the rate of 40 liters per minute. The silane addition was 0.22liter per minute for 1.5 hours and then increased to 0.31 liter perminute for another 1.5 hours. The addition of natural gas to the wastestream containing silane was at the rate of 4.7 liters per minute. As inother tests, the foraminous burner was fired at the rate of 44,000 BTUper hour per square foot with 61% excess air. The silane was completelydestroyed and the resulting silica particles did not accumulate in thedestructive combustion zone.

Additional tests were carried out on waste streams containing CF₄ aloneand mixed with C₂ F₆ and silane. The desired destruction of thefluorocarbons and silane was again accomplished. The efficacy of thecombustive destruction of halogenated compounds and/orparticulate-forming matter has been established and shown to beattainable by simple adjustments in the amounts of fuel gas fed to theforaminous burner and added to the waste stream containing troublesomesubstances even when the injection rate is varied over a wide range.

Simple tests have also provided guidelines for selecting for any wastestream the conditions that will yield in excess of 95% destruction ofhalogenated compounds which are resistant to breakdown. The fuel gassupplied to the foraminous burner should be at a rate of at least about25,000 BTU per hour per square foot of burner surface. Combustion air issupplied solely to the burner in an amount that exceeds thestoichiometric requirement of both the fuel gas supplied to the burnerand all the combustibles injected into the destructive combustion zone;a large air excess of at least about 50% relative to the fuel gassupplied to the foraminous burner is a good starting level in seekingthe optimum conditions for any particular waste stream containinghalogenated compounds and/or particulate-forming matter. The amount offuel gas on a BTU basis supplied to the foraminous burner is alwaysgreater than that admixed with the waste stream. The ratio of burnerfuel gas to waste stream fuel gas is usually in the range of about 2:1to 5:1, high ratios being generally applicable to low rates of wastestream treatment and low ratios being generally applicable to high ratesof waste stream treatment. These guidelines facilitate both theselection of the initial conditions for treating a particular wastestream containing halogenated compounds and/or particulate-formingmatter, and then the adjustment of these initial conditions to attain adesired high level of halogenated compound and/or particulate-formingmatter destruction such as 99%.

The invention is noteworthy for the simplicity and compactness of thefurnace as well as for the simplicity of operation and low fuelconsumption to easily effect 99% destruction of halogenated compoundsand/or particulate-forming matter such as silane and arsine, whileavoiding the troublesome deposition and accumulation of particulatessuch as SiO₂ and As₂ O₃ within the combustion zone.

Those skilled in the art will readily visualize variations andmodifications of the invention in light of the foregoing disclosurewithout departing from the spirit or scope of the invention. The term"foraminous gas burner" is used herein to include not only both porousand perforated burners but also such burners in unitary form as shown inFIG. 3 as well as in modular form as shown in FIG. 2. The many patentsdealing with foraminous burners are certainly suggestive of manypossible variations. Accordingly, only such limitations should beimposed on the invention as are set forth in the appended claims.

What is claimed is:
 1. A process for the combustive destruction ofparticulate-forming hydrides and/or halogenated compounds selected fromthe group consisting of halogenated hydrocarbons, fluorocarbon gases,nitrogen trifluoride and sulfur hexafluoride, in a combustion zonelaterally surrounded by the exit surface of a foraminous gas burner,which comprises injecting a stream containing at least one of saidhalogenated compounds and/or particulate-forming hydrides and added fuelgas into the top of said combustion zone, simultaneously supplying fuelgas and air to said foraminous gas burner to effect combustion at saidexit surface, the amount of said fuel gas simultaneously supplied tosaid foraminous gas burner being, on a BTU basis, greater than that ofsaid added fuel gas, and the amount of said air being in excess of thestoichiometric requirement of all combustibles entering said combustionzone, and discharging the resulting combustion product stream from thebottom of said combustion zone.
 2. The process of claim 1 wherein theratio of the amount of fuel gas supplied to the foraminous gas burner tothe amount of fuel gas injected into the combustion zone is in the rangeof about 2:1 to 5:1, and said amount of fuel gas supplied to saidforaminous gas burner yields upon combustion at least 25,000 BTU perhour per square foot of the exit surface of said foraminous gas burner.3. The process of claim 1 wherein all of the fuel gas consumed by saidprocess is natural gas and the amount of air supplied to the foraminousgas burner is at least about 50% in excess of the stoichiometricrequirement of said natural gas supplied to said foraminous gas burner.4. The process of claim 1 wherein the stream is nitrogen containing atleast one fluorocarbon and/or a hydride discharged as a waste streamfrom the manufacture of semiconductors.
 5. The process of claim 4wherein the combustion product stream is discharged downwardly from thebottom of the combustion zone directly into a column while cooling waterflows down the inner surface of the walls of said column.
 6. The processof claim 5 wherein the fuel gas supplied to the foraminous gas burner isnatural gas in an amount to yield upon combustion at least 25,000 BTUper hour per square foot of the exit surface of said foraminous gasburner.
 7. The process of claim 6 wherein the fuel gas added to thestream is natural gas, and the ratio of the amount of natural gassupplied to the foraminous gas burner to the amount of natural gasinjected into the combustion zone is in the range of about 2:1 to 5:1.8. A process for the combustive destruction of particulate-forminghydrides and/or halogenated compounds selected from the group consistingof halogenated hydrocarbons, fluorocarbon gases, nitrogen trifluorideand sulfur hexafluoride, in a combustion zone laterally surrounded bythe radiant surface of a foraminous gas burner, which comprisesinjecting a stream containing at least one of said halogenated compoundsand/or particulate-forming hydrides and admixed fuel gas into the top ofsaid combustion zone, said burner being supplied with fuel gas andexcess air to yield at least 25,000 BTU per hour per square foot of saidradiant surface, controlling on a BTU basis the ratio of the amount ofsaid fuel gas supplied to said foraminous gas burner to the amount ofsaid fuel gas admixed with said injected stream in the range of about2:1 to 5:1 to achieve the combustive destruction of at least about 95%of the content of said halogenated compound and/or particulate-forminghydride in said injected stream, and discharging the resultingcombustion product stream from the bottom of said combustion zone. 9.The process of claim 8 wherein the combustion product stream isdischarged downwardly from the bottom of the combustion zone directlyinto a column while cooling water flows down the inner surface of thewalls of said column.
 10. The process of claim 9 wherein the injectedstream is nitrogen containing at least one fluorocarbon and/or ahydride.
 11. The process of claim 10 wherein all of the fuel gasconsumed by said process is natural gas and the amount of air suppliedto the foraminous gas burner is at least 50% in excess of thestoichiometric requirement of the portion of said natural gas suppliedto said foraminous gas burner.